In order to reduce the amount of unburned hydrocarbons, carbon monoxide and nitrogen oxides (generally NO.sub.x, principally NO and NO.sub.2) in automotive exhaust gas to acceptable levels, special exhaust treatment catalysts and internal combustion engine operating practices have been developed. The successful conversion of the exhaust gas constituents is complicated by the requirement of simultaneously and rapidly oxidizing carbon monoxide to innocuous carbon dioxide and reducing NO to nitrogen. In general, of course, the oxidation reaction is favored by an abundance of oxygen, whereas the reduction reaction is inhibited by it. It has been found that when the proportions of air and gasoline are balanced close to their stoichiometric ratio, the simultaneous oxidation of CO and unburned hydrocarbons and the decomposition of NO can be accomplished to an acceptable degree. Catalysts called three-way catalysts have been devised for these operating conditions. In general, such catalysts comprise a dispersion of very small particles of platinum and rhodium on a carrier of high surface area particulate alumina.
These noble metals are scarce and expensive. They occur together in ores. Unfortunately, the present three-way catalysts require rhodium in an amount that exceeds its natural proportion in the noble metal bearing ores. It is, therefore, desirable and an object of this invention to devise and provide a catalyst that requires lower noble metal content in general and less rhodium in particular. It is, of course, necessary to also maintain or improve the efficiency of the automotive exhaust gas treatment.
As stated above, it is a current practice to operate automotive engines at approximately the stoichiometric air-fuel ratio. Because of varying gasoline compositions and rapidly changing engine operating parameters, it is not practical to operate the engine air-fuel ratio precisely at the stoichiometric proportion. Instead, it is the practice to continually and frequently cycle the air-fuel ratio of the mixture burned in the cylinders of the engine from just fuel rich of stoichiometric to just fuel lean of stoichiometric.
Such cyclic variation of air-fuel ratio is made possible by an oxygen sensor located in the exhaust manifold which detects whether the engine is then operating rich or lean of the stoichiometric ratio. The exhaust gas contains a very low amount of oxygen when the engine is running fuel rich and a much higher concentration of oxygen when the engine is then operating fuel lean. Continued signals from the oxygen sensor are processed by the engine computer control system to vary the fuel input to the engine such that it cycles between rich and lean virtually every second. Such rapid cycling of the air-fuel ratio produces an exhaust gas that over relatively short time increments averages stoichiometric air-fuel engine operation. The catalyst responds to such cycling of the air-fuel ratio. Therefore, it is desirable to have a catalyst which would permit more latitude in engine operating conditions and less frequent cycling of the fuel control and supply system. It is an object of this invention to provide such a catalyst.